Latest Publications

Environmental Physics 2014 video competition results

By Matt Owens

Yesterday (Wednesday 26 November) was the screening event for the Environmental Physics 2014 video competition, in which we asked GCSE and A-level students to put together a one minute video describing an aspect of the physics of light.  The topic was chosen in support of UNESCO’s upcoming international year of light event.

The judges with second place winner, William Tyrrell

The entries covered a huge spectrum of topics [groan – Ed], as showcased below, from students all over the UK. The judging panel comprised a space physicist, a stratospheric physicist and a scientific outreach expert (with a murky past in astrophysics).  Using the new video wall facility, the panel and audience considered five shortlisted videos. After much deliberation (and cake), the following winners were announced:

First place:

Embla Hocking (Exeter Mathematics School) for her video, “Why can we see pink?”

Prize: iPad Mini and a cloud chamber for detecting cosmic rays to her physics department

Second place:

William Tyrrell (King’s College, Wimbledon) for his video, “Light speed

Prize: £100 of Amazon vouchers

Third place:

Philippa Copeland (Havant Sixth Form College, Southampton) for her video, “Lenses

Prize: £50 of Amazon vouchers

Runners-up:

Mo Awe (Putney High School) and Alex Bytheway (Rydal Penrhos, North Wales) for their entries on “Gamma Rays” and “Photosynthesis,” respectively.

Prize: £20 of Amazon vouchers.

A big thank-you to everyone who took part and we hope for an equally successful competition in 2015!

Matt Owens


The end of the rainbow … an open letter to the climate science community

An open letter to the climate science community

By Ed Hawkins, Doug McNeall, David Stephenson, Jonny Williams & Dave Carlson

Dear colleagues,

This is a heartfelt plea.

A plea to you all to help rid climate science of colour scales that can distort, mislead and confuse. Colour scales that are often illegible to those who are colour blind.

The main culprit is, of course, the ‘rainbow’:

We have all likely used it, and we have all certainly seen it – presentations, posters, papers, blogs and news articles full of figures with similar colour scales.

However, the most commonly used rainbow colour scales can distort perceptions of data and alter meaning by creating false boundaries between values. There are numerous blogsand published papers from visualisation experts illustrating these issues. In one example, changing to a non-rainbow scale even improved accuracy of heart disease diagnoses.

And, if you use a rainbow colour scale, you will have a friend or colleague that is colour blind and may confuse the colours.

This is not the first such plea.

A decade ago an article appeared in EOS, demonstrating that contrasting red with green can render a figure illegible to the 8% of the male and 0.4% of the female population who are colour blind. The EOS article suggested that journals should do more to improve the colour accessibility of figures.

But, the problem is now worse than a decade ago. Most issues of every major climate journal have figures which are potentially misleading and colour inaccessible. Maps, line graphs and histograms can all have confusing colour combinations.

Journals, rightly, do not tolerate poor grammar, incorrect spelling, or muddy descriptions of scientific methods. It should be no different for visual communication. We should be equally intolerant to poor use of the grammar of graphics as we are to its written equivalent.

It is not just the journals who need to act. As scientists increase their efforts to make their work accessible to the public through the media, blogs and social media, there are more opportunities to show poor figures.

What are the possible solutions?

We need to be more willing to discuss and criticise the visualisation of the science as well as the science itself.

Authors should be responsible about the colour choices they make. Journals might add colour accessibility to their existing guidelines for acceptable figure types. Reviewerscould recommend revision if such colour scales are used. Editors should not accept papers which use inaccessible and potentially misleading colour scales. And, the mediamight reconsider using such figures from published work.

We know ‘rainbow’ is the default colour scale in many commonly used programming languages, but that doesn’t make it the best. Resources are easily available to change colour scales for RIDL (& here), MATLABPython.

There are numerous websites and online tools giving advice and recommending safe and better colour scales (such as Color Brewer or HCL Wizard). You can even test online how your figures might appear to those who are colour blind. Adding different shape markers in line graphs might also aid interpretation.

Choosing a good colour scale is not difficult – it just takes awareness and a few moments of effort. The best choice will probably depend on the situation, so ask yourself why you have chosen that particular colour scale.

We take heart from some recent progress.

The journal BAMS recently took a step forward by publishing an article pointing out the flaws with rainbow colour scales. MATLAB have just announced that they are changing the default rainbow colour scale, giving a comprehensive explanation considering colour accessibility and perception issues.

All of us could do more in improving the clarity of our figures, the authors of this open letter included. More needs to be done. And, it needs all of us to do more.

So, we undertake this pledge – to never again be an author on a paper which uses a rainbow colour scale.

If you agree to also make this pledge (or disagree), please comment below this post. Oremail us. And tell your colleagues.

We hope that you will join us.


We encourage the climate science community to communicate this letter widely. To spread the word on twitter, please use #endrainbow. Short URL: http://tiny.cc/endoftherainbow

Other climate-related case studies:

Example of simulating colour blindness with different colour scales & MATLAB software:Which colour scale is best for you?

Considering different colour scales for sea-level change maps: Better palettes

And, it is not just climate. One of the iconic images in astronomy – the Cosmic Microwave Background (CMB) – is normally in rainbow


Many thanks to all those who have patiently commented on these issues.

Space weather – sunspot AR2192

By Simon Thomas

Friday 24 October 2014 – updated 30 October 2014

Sunspots are areas on the Sun which appear dark in contrast to the solar disk. They are associated with complex magnetic fields which inhibit convection and are therefore not as hot as the surroundings (sunspots are typically 3000-4500 K compared to the surrounding at, on average, 5780K). The magnetic fields in sunspots are arranged vertically and are either pointing inwards or outwards. Therefore, when we discuss “sunspots” we are typically referring to sunspot-pairs, where the magnetic field points out of one, then loops and connects inwards to other sunspot.

Active regions on the Sun, including sunspots, are of interest to us as they are effectively source regions for solar flares and coronal mass ejections (CMEs). Solar flares are bright flashes interpreted as a huge release of energy. The emitted x-ray and ultraviolet (UV) radiation from solar flares can affect satellite communication systems at Earth. A flare is often, but not always followed by a CME, a subsequent eruption of plasma and magnetic field out from the Sun and into space. If a large CME erupts and impacts Earth, then it can trigger “geomagnetic storms” which can result in low-latitude aurora, disruption to power grids and enhanced radiation doses to humans at high altitude. Full space weather hazards and risks are discussed by Hapgood (2010).

The large sunspot AR2192, which has made the news this week, was not too unusual when it was last visible before rotating around the opposite side of the Earth. As our only spacecraft which view the Sun on the other side of the Sun are now turned off while the Sun is between them and Earth, we have not been able to follow the sunspot through its progression until recently. However, since it has rotated around and become visible to ground-based and observations from the Solar Dynamics Observatory, we have been able to observe and analyse its progression and development. See this link for a video clip of the progression of this sunspot from youtube.

So, why has sunspot AR2192 caused excitement within the media? Well, firstly it has just been confirmed to be the largest to be observed in almost 25 years. It is by far the largest sunspot in this current Solar Cycle and has been visible to the naked eye (with the help of filters – don’t look directly at the Sun!). This interest has been enhanced as it coincided with a recent partial eclipse viewable from the USA. To be able to comprehend the size of such a sunspot, as the pictures on the solar disk do not give it justice, we can compare with the sizes of the planets. This is shown in Figure 1 where the sunspot on the 22 October is shown to scale to be approximately the size of Jupiter, and 14 times the size of Earth.

Figure 1 – Sunspot AR2192 on 22 October 2014 compared to the sizes of Jupiter and Earth. Image courtesy of www.thesuntoday.org.

Since this image was taken, the sunspot has grown larger still.

The second point relating to our interest in this sunspot is in the history of such sizeable sunspots. The last time we saw a sunspot of comparable size, it was associated with consecutive Earth-directed CMEs known as the “Halloween Storms” which took place in 2003. These caused aurora to be observed as far south as Texas and the Mediterranean, and caused a power outage in Sweden.  There have, however, been larger sunspots since records began. Figure 2 shows a graph of the largest yearly sunspots of the 20th Century. The largest peak on here is the so called “Great Spot of 1947”. This grandly-named sunspot was the largest recorded, but in contrast to the 2003 event that produced the Halloween Storms, this, it appears, did not produce a large Earth-directed CME. This does not, however, mean that it did not release a huge CME in a different direction as it rotated around, but it is unfortunate that we have only had a means of observing remote coronal mass ejections and flares in recent years.

It is worth discussing now that just because a sunspot is large, this does not necessarily mean it is capable of producing hazards to us on Earth. Firstly, the active region (including the sunspot) must have suitably complex and compressed magnetic fields to release a CME in the first place. Secondly, even if the active region is capable of producing CMEs, the eruption of this CME would have to be at the correct angle to intercept Earth. CMEs in the solar system are fairly large structures, but Earth is a small target compared to the number of possible trajectory angles of the CME.

Figure 2 – Largest yearly sunspots observed from 1900-2000 in millionths of the total solar disk. Courtesy of David Hathaway, NASA.

So, with that said, what are the prospects of getting some significant space weather from AR2192? Currently, the new Met Office space weather prediction centre have been keeping a close eye on the evolution of this sunspot and this is a sizeable, early challenge for their forecasters. The sunspot group has been producing a high frequency of powerful solar flares, but as of yet, no significant CMEs in our direction. This led the Met Office to release the following forecast on 23 October 2014, for the weekend: “Solar activity is likely to remain at moderate to high levels with a chance (30%) of X-class flares. Geomagnetic activity is expected to remain elevated as a further coronal hole high speed stream becomes geo-effective but with only a slight chance (15%) of minor storms”. The coronal hole part relates to a large, slightly darker region of the Sun ahead of AR2192 which has been releasing faster solar wind than the ambient wind speed.

The predictions from the Met Office and a similar forecast from NOAA show that it is very likely that there will be further solar flares, perhaps up to the most intense, X-class of flare. However, it does appear that there is a low chance of a CME impacting in the next ~3 days. However, this does not mean that this would not happen in the slightly longer term; CMEs generally take a matter of days to reach Earth and so as the sunspot is now Earth-directed, a CME associated with an anticipated flare may not reach us until early next week. Such a flare with an associated CME has not occurred, as of 1500 UTC on Friday 24 October.

Finally, let’s think about the longer term. Are we likely to see any more sunspots of this size in the coming years and what is the chance of a large Earth-directed CME? The sunspot cycle has just peaked at solar maximum and solar activity is starting to decline. As the maximum sunspot size roughly follows the sunspot cycle, it is unlikely that we shall see another of this size for a good few years, but not impossible! Secondly, solar activity in general appears to be in decline from the very large cycles we saw in the late 20th Century. The latest solar cycle has been very weak compared to previous cycles in both sunspot numbers as well as solar wind parameters such as the magnetic field in near-Earth space. This is shown in the top two panels of Figure 3 (from Lockwood et al., 2012). These show the sunspot number (R) in the top panel and the near-Earth magnetic field in the second. The black lines in these are the data and it is apparent that the number of sunspots has began to decline here and this has coincided in the magnetic field drop.

Figure 3 – Data and predictions from Lockwood et al. (2012) of sunspot number (R; top panel), the near-Earth magnetic field strength (B; second panel), cosmic ray number from a station in Finland (Onm; third panel), and the aa index, showing changes in the Earths magnetic field (aa; final panel). Black lines are raw data, purple lines are reconstructions and red to blue are likely patterns that the observations will follow.

With the number of sunspots reducing, it is unlikely that there will be as frequent “super-sunspots” as were seen during the last century. Moreover, the predictions from Figure 3 (based on predicted variations from previous scenarios in ice-core data) show that sunspot number is likely to reduce even further with weaker magnetic fields. Thus, although solar activity is very difficult to predict, sunspot AR2192 has given us a rare opportunity to study the evolution and activity of such a sizeable sunspot, which will hopefully give a useful and significant contribution to future space weather forecasts.

UPDATE (30 October): As AR2192 is disappearing around the east (right-hand) limb of the Sun, it is still increasing with size. It has produced a large number of M- and X- class solar flares but no sizeable coronal mass ejections, which was as anticipated by the Met Office prediction centre. Sunspots can persist for several solar rotations (which take approximately 27-days to complete), and so it is possible that we shall see the active region again in a few weeks.

REFERENCES

Hapgood, M.A., 2010. Towards a Scientific Understanding of the Risk from Extreme Space Weather. Adv. Space Res., 47, 2059–2072

Lockwood, M., M. J. Owens, L. Barnard, C. J. Davis, and S. R. Thomas, 2012. What is the Sun up to? Astron. and Geophys., 53, 3.9–3.15

An analogue forecast for winter 2014/15 in Reading

By Roger Brugge

Reluctant as I am to do long-range predictions, analysis of Reading data for the period 1961-2010 suggests the following:

Rainfall – 2014 has given us a dry September and (already) a wet October. Let’s assume that overall autumn rainfall is close to normal. After an autumn with normal rainfall the likelihood of winter being

  • Wet/very wet  is 17%
  • Normal: 50%
  • Dry or very dry: 33%

October has been a dull month so far after near-normal September sunshine. Overall the autumn may well have close to normal sunshine. After a ‘normal’ autumn we find that winter sunshine was as follows:

  • Sunny or very sunny is 23%
  • Normal : 57%
  • Dull or very dull: 20%

Finally, autumn has so far been a warm season with a mild October after a warm September. Warm or very warm autumns are followed by winters that were

  • Mild or very mild is 56%
  • Normal: 12%
  • Cold or very cold: 32%

So, a tongue-in-cheek forecast at this mid-way point in October would be for a winter in Reading that is close to normal in terms of rainfall and sunshine, but probably milder than average.

17 October 2014

Tornadoes in the UK?

By Tom Frame

Last Wednesday (8 October) saw reports of several tornadoes in the UK – one even tore the roof off a house. I remember whengrowing up I always thought of tornadoes as something that occurred only in the USA – perhaps a school production of The Wizard of Oz had put this in my mind. So a few years ago (probably more than a few now), the first time I ever heard of tornadoes in the UK, I was pretty shocked. But just how common are these?

When I asked around I was told that it is often said that the UK has the highest number of tornadoes per year per square kilometre in the world. Is this really true? Interestingly a study published in 2003 (reference 1) seems to suggest that the UK has a fairly high number – but the highest? No …
It turns out that if you go by actual number of tornadoes observed , then the Netherlands has the most, Estonia second, Republic of Ireland third, the UK fourth and the USA fifth. Whereas if you go by an estimate of the true number then the UK jumps up to second place behind the Netherlands.
So tornadoes in the UK – not a big surprise.
The real reason that the USA is strongly associated with tornadoes is that it has the most intense and damaging tornadoes in the world. So whilst the UK has many, they are weak and short-lived usually causing only minor damage.
Reference 1
Dotzek, Nikolai. “An updated estimate of tornado occurrence in Europe.” Atmospheric Research 67 (2003): 153-161.

The ozone layer shows first signs of recovery, but …

By Michaela I. Hegglin

Just over two weeks ago, the United Nations (UN) held a press conference in New York to announce the release of the Assessment for Decision Makers (ADM), a summary document of the WMO/UNEP Scientific Assessment of Ozone Depletion 2014. The report is the work of a UN panel of 300 scientists from around the world (including four scientists from our Meteorology department) and represents the latest comprehensive update on the state of the Earth’s ozone layer, which protects the Earth from the Sun’s harmful ultraviolet radiation.

Figure 1: The ozone layer is on its way to recovery. The past evolution of ozone observations is well understood and can be modeled by complex chemistry-climate models (CCMVal-2 simulations in black with uncertainty in grey). Pink curve shows the potential of ozone-depleting substances to destroy ozone in the stratosphere (EESC; pink). Other colors show ozone evolution for different representative pathway scenarios (RCPs). (Source: ADM, WMO/UNEP ozone assessment, 2014).

The encouraging finding of this year’s Assessment is that the ozone layer is showing first signs of recovery. A reduction in ozone depletion is expected given the decline in stratospheric chlorine abundances by 10-15% since peak values in the late 1990s/early 2000s. However, the detection of ozone recovery has been anything but trivial, since the signal has to be disentangled from natural variability, enhanced ozone depletion after the Mt Pinatubo volcanic eruption in 1991, increases in tropospheric ozone, and the impact of climate change, all of which affect total column ozone in addition to ozone-depleting substances. While the observed increase in total column ozone seen in Figure 1 is consistent with model predictions, the authors of the Assessment were not ready to attribute it with high confidence to the decline in ozone-depleting substances (although a subsequent study by University of Reading authors has now done so*). Therefore the Assessment states that there are ‘indications of recovery’ and not yet recovery itself.

Astonishingly, there are skeptics who deny ozone depletion has ever happened. Reactions such as ‘Oh, so the ozone hole was just yet another scare-story of the environmentalists, such as acid rain and climate change?!’ were to be heard days after the ADM-release. Well, we definitely know better. If the world would not have reacted quickly to the threat from ozone-depleting substances, we would be in serious trouble now. At the time of the Montreal Protocol, ozone-depleting substances were set to grow at a rate that by today would have caused at least twice the ozone depletion that we have experienced (see Figure 2). Stopping and reversing the growth of the atmospheric concentrations of ozone-depleting substances has thereby helped avert an estimated 2 million skin cancer cases per year by the year 2030.

Figure 2: The world avoided. Without the Montreal Protocol, the global ozone layer would have experienced serious depletion around the globe. The change from red in 1989 to green-yellowish colours in 2028 indicate a thinning of the ozone layer by around 20%  (credit: Paul Newman, NASA Goddard, USA)

The bad news… Although the Montreal Protocol has averted the worst outcome of ozone depletion, the ozone layer is never expected to return to a pristine state. The culprit is climate change as discussed in the ADM.  Models predict that the ozone layer will be strongly affected by greenhouse gases due to both physical and chemical mechanisms. Nitrous oxide (N2O) and methane (CH4) both affect ozone chemically. Carbon dioxide (CO2) cools the stratosphere and also leads to increases in stratospheric ozone due to a slowing down of chemical loss reaction rates, ironically ‘helping’ the ozone layer to heal. Finally, the combined greenhouse effect of these gases increases the strength of the stratospheric overturning circulation. The strengthened circulation leads to a decrease in total column ozone in the tropics, and to an increase in the extratropics, the extent of which is dependent on the greenhouse-gas scenario the world will follow into the future (see Figure 1). However, too little UV radiation (as a consequence of a thicker ozone layer) can have adverse health impacts too, especially at higher latitudes where it is well known to lead to Vitamin-D deficiency and ailments such as rickets.  But because the ozone decrease expected from climate change will be in the tropics, it may this time affect people who are least educated about the ozone layer, and who have the least means to protect themselves.

The work is not done yet, moreover, since the substitute gases currently used by industry to replace the ozone-depleting substances are themselves strong greenhouse gases and if left unabated may contribute 10% or more to the climate forcing from CO2 by the year 2050. The problem is identified and industry seems to be at least prepared to investigate solutions. The latter was a key aspect throughout the process of the Montreal Protocol: bringing all stakeholders to the table — scientists, industrialists, economists, legal experts and policymakers — to find a compromise on how to deal with this global environmental issue. It took almost 30 years before the policy action has borne fruit. But full recovery of the global ozone layer is not expected before the middle of the century, and even later in the Antarctic –  a reminder of how long environment damage can accompany us. Even more so, the Montreal Protocol should give us hope that humankind is able to tackle climate change, and its mechanisms may well serve as an example of how to do it (see also commentary in the Guardian http://www.theguardian.com/commentisfree/2014/sep/11/ozone-layer-recovering-global-treaty-chemicals-fossil-fuels).

http://www.nature.com/ngeo/journal/v7/n6/full/ngeo2155.html

A wet winter – and what might come next?

Much has been written/spoken in recent weeks about the amount of rain that has fallen this winter (December-February) in southern England (and other regions) of the United Kingdom. But several other interesting facts appear to have been overlooked in the almost daily (at times) deluge of information above high winds, heavy rainfall, high tides and low pressure. Here I attempt to detail some of these from a Reading perspective.

I begin with a look back at the conditions in February and then at the winter as a whole. Finally I present a tongue-in-cheek look ahead at spring and summer, using an analogue forecasting approach.

The weather observations referred to here date back to 1908 when the forerunner of the University of Reading began climatological measurements at a campus off the London Road close to the town centre. In 1968 measurements were transferred to the cooler site at Whiteknights; this change needs to be remembered when examining long-period temperature records although the very small difference in rainfall statistics between the two sites can be safely ignored.

Conditions during February

February rainfall totals at the University of Reading.

117.2 mm of precipitation fell in Reading during February, making it the wettest February on record at the University. It was only the fifth February since 1908 to record at least 100 mm, the total fall amounting to almost three times the normal February fall of 40.9 mm. 24.9 mm fell on 6 February, making this the fourth wettest February day in the Reading record.

This rainfall (no snow was recorded during February in Reading) was the result of a continuation of the passage of low pressure systems across the British Isles that had been experienced during the preceding six weeks. In fact the average mean sea level pressure at the University during February was just 997.6 mb – the lowest on record there for any month. Initial studies suggest that this value is lower by 1.1 mb than the lowest value recorded for any month in London’s records back to 1692 (Cornes et al., 2011). Typically the air pressure in London might be expected to be about 0.5 mb greater than that in Reading.

As a result of these generally cyclonic conditions, winds during the month blew from a southerly or south-westerly direction for much of the time. During the stormier interludes peak gusts reached 54 knots (62 mph) on the 14th with 40 knots (46 mph) being reached on four other days.

With winds from a mild south-westerly direction prevailing throughout the month, February was much milder than average. The average temperature overall of 6.9 °C was 2.1 degC above average making it the warmest February (along with 2011) since 2002.

Despite all the rain and associated cloud cover, February was a sunnier month than normal. The University’s sunshine recorded notched up a total sunshine duration of almost 99 hours, making it the sunniest February for six years.

Minimum air temperature in February

The lowest temperature recorded at the University during this February was a relatively balmy 1.3 °C on the 16th, meaning that Reading (along with much of southern England and East Anglia) was free of air frost this month. This was the warmest ‘coldest February night’ since 1.8 °C in 1961 (albeit this was at the slightly warmer London Road site). Only in 1961, 1966 and 1990 has February been free of air frost before this at the University. Normally in February we would expect the lowest minimum temperature to be -4.5 °C. There was, however, an air frost on 1 March.

Frost-free spells in winter

With Reading being air frost-free, how does this compare with other spells that failed to record an air frost at this time of year? The last day of February was the 31st consecutive day with temperatures remaining above 0 °C – spells of this length (and longer) have occurred during eight other winters in records at Whiteknights that date back to 1968; three of these have been since 2004 while at the slightly warmer site at London Road a remarkable 57-day spell ending 19 March 1966 was frost-free.

Wettest winters and seasons

Winter rainfall at the University, 1908/9 to 2013/14.

With 374.6 mm of rain falling this winter, it has been the wettest winter (and, indeed, season of any name) in the University’s rainfall record. Previously, the wettest winters had been those of 1914/15 (327.9 mm) and 1989/90 (344.6 mm) – these being the only winters to record over 300 mm of precipitation. The only seasons in which 350 mm or more precipitation had previously fallen were the autumns of 2000 (353.4 mm) and 1974 (352.2 mm). Much of this winter rain fell in just 11 weeks, and the winter’s fall represents 59 per cent of the annual average rainfall at the University.

‘5 mm+’ days

The incidence of days with 5 mm or more precipitation during winter, 1908/9 to 2013/14.

After a dry start to December (only 4.7 mm of rain fell during the first fortnight of the month) this winter’s rain has tended to fall frequently with moderately large daily totals, rather than being due to a few exceptionally wet days. The wettest day of the winter was a fall of 29 mm of rain fell on 23 December. However, the number of days when 5 mm or more of rain fell totalled 28 this winter – more than in any other winter in the University’s record. In fact only the only time when as many wet days occurred in three consecutive months was during October to December 1929 and November 1929 to January 1930 – both these periods also had 28 wet days.

Winter temperature and sunshine

Each month this winter was warmer than normal, with the average temperature for the season of 6.5 °C making it the mildest winter since 2005/6 when 7.0 °C was the average temperature. The lowest temperature recorded this winter was -3.8 °C on 12 January – this was the only morning when the air temperature reached -2 °C or lower.

Only in ten winters has the air temperature remained above -3.8 °C since 1908 – at the current (colder) Whiteknights site this has happened only in 1974 (when the lowest air temperature was -2.2 °C), 1988 (-3.1 °C) and 1997 (-3.4 °C).

The incidence of ground frost (when the temperature at grass tip level falls below 0 °C) was close to normal this winter.  However, air frosts were infrequent throughout the winter – only on 10 mornings did the air temperature drop below 0 °C, the fewest occasions since just 8 mornings in 1989/90. The only other winters since 1908 to record fewer air frosts were those of 1960/61 and 1974/75 – both of these had 9 air frosts.

Only one day with slight sleet (during a short-lived shower) was observed at the University this winter. Although this incidence of snow/sleet is much less than during the previous five winters; in Reading it is not uncommon for winters to have just one or two days of snow – or indeed, to be free of any snowfall at all.

This winter has also been sunnier than average with 198 hours of sunshine being recorded – although this was not as sunny as in 2011/12.

Winter air pressure and winds

Air pressure this winter was been generally low after mid-December with frequent stormy spells in Reading. The average air pressure of 1004.7 mb was the lowest for a Reading winter since 1914/15 – which was also another very wet winter. The peak gust occurred on Christmas Eve in the current winter – when 76 mph was recorded.

Forecasts based on analogue conditions after previous wettest winters

While seasonal forecasts nowadays tend to be made using numerical weather prediction (i.e. by means of computer models of the atmosphere and ocean), before the advent of such models an analogue method was employed. This consisted of finding similar months/seasons to the ones that had just occurred and then examining the weather conditions that had followed these similar (i.e. analogue) months. This typically meant examining the large-scale flow atmospheric patterns as well as local (i.e. UK-wide) conditions.

The analogue method has a major drawback in that it is virtually impossible to find a perfect analogue pattern from the past. This is particularly true when recent conditions have been so extreme as to have fallen into the ‘wettest on record’ category (as in this winter) – implying such extreme conditions have not occurred previously in the instrumental record. In particular, an analogue forecast of seasonal UK weather should take into account conditions over much of the Northern Hemisphere, if not global conditions, over recent weeks.

Accepting this drawback, it is nevertheless interesting to examine what types of spring and summer weather have followed previous winters in Reading that have been this wet or mild.

During the 50-year period 1961-2010 out of 20 mild or very mild winters, 12 were followed by a mild or very mild spring and just two following springs were cool or very cool. As mentioned above the winter of 2005/6 was a mild winter – and the summer of 2006 was the warmest summer on record in Reading.

Prior to this year, by far the wettest winters since 1908 in Reading were those of 1914/15, 1989/90 and 1994/95. Each of these three winters listed above were followed by a dry or very dry spring.

  • The spring of 1915 had two notable droughts when about 38 mm of rain fell in almost ten weeks from March to mid-May. Late May was a little wet but then June was dry and July turned wet.
  • In the spring of 1990 just 39.2 mm of rain fell at the University making it the driest spring on record in Reading since 1908. The late summer of 1990 was hot (reaching 35.5 °C in August) and sunny and after February it was a very dry until December.
  • August 1995 was hot and dry and the summer was generally a warm one. The period April to August was remarkably dry with just 3 mm of rain and almost 280 hours of sunshine in August made it the second sunniest August on record.

In addition, the winter of 2002/03 was another winter with lots of flooding locally around the start of January – and that had a hot, dry August with 30 °C reached on 10 days during the summer, including 36.4 °C on 10 August.

Maybe this is just nature’s way of saying that nothing lasts forever? But note also that there have of course, given the caveats mentioned above, been other very wet (but not quite as wet) winters locally after which the spring and summer months have not been so benign (for example the summer of 1877 when late May frosts in Berkshire created havoc with budding potato plants and only June was dry, and the spring/summer of 1912 when (after an almost totally dry April in east Berkshire) the summer was said to be one of the wettest for 50 years).

Fingers crossed then!

Acknowledgements

The daily weather observations during the current winter have been made, almost exclusively, by the University’s meteorological observer Mike Stroud.

References

Cornes, R. C., Jones, P. D., Briffa, K. R. and Osborn, T. J. (2011) A daily series of mean sea-level pressure for London, 1692-2007. International Journal of Climatology. doi: 10.1002/joc.2301

December 2013-January 2014 in Reading: Wet and, at times, stormy

A brief overview

After a dry start to December 2013 (just 11.3 mm of rain fell in the 33 days ending 0900 UTC on 15 December at the Department’s weather station) there was an abrupt change in the weather that was to last for over three weeks.

Many others have, over recent days, written and spoken about the reasons behind this wet and stormy period of weather in the UK. Here, I shall present some local statistics and try to place the event into an historical perspective.

Wind and pressure

Numerous deep depressions, guided towards the UK by an unusually vigorous jet stream, were to bring frequent gales and heavy rainfall to many parts of the British Isles, most notably to southern and western areas, from mid-December to early January.

In Reading peak wind gusts recorded during the period 0000-2400 UTC included the following

23 December: 52 knots,          24 December: 66 knots,          27 December: 45 knots,
30 December: 40 knots,          3 January: 45 knots.

The gust on the 24th was the strongest gust in Reading since February 1990 when 71 knots was measured. The highest gusts in the Reading wind record remain those of 76 knots on 25 January 1990 (the Burn’s Day storm) and 75 knots on 2 January 1976. Gust records date back to 1961 in Reading (although there is a gap in the record during the period August 1971 to May 1974).

Met Office surface analysis, 1200 UTC 24th December 2013

The conditions In Reading on 24th December were due to an exceptionally deep depression centred to the west of the British Isles. The analysed central pressure at 1200 UTC was 927 mb to the northwest of the Hebrides; the mean sea level pressure of 936.4 mb recorded at Stornoway Airport at 1229 GMT was the lowest over any land in the British Isles for any month since 1886 (see Burt SD. 1983. New UK 20th century low pressure extreme. Weather, 38: pp 208–213 and Burt SD. 1983. The lowest of the Lows … Extremes of barometric pressure in the British Isles, Part 1 – the deepest depressions. Weather, 62: pp 4-14).

The winds overnight 23rd-24th December did cause some local damage – primarily downed roof tiles and some trees, but also other building damage in central Reading, e.g. ‘The Blade’ building.

In Reading over the Christmas-New Year period MSL pressure fell to the following low values as depressions passed close by:

24 December 973.3 mb
27 December 981.0 mb
1 January 982.5 mb

On the 24th, the contrast between the minimum in Reading and that mentioned at Stornoway Airport is remarkable and explains the wind strengths observed across the British Isles on the 23rd-24th.

With weather systems bringing our weather from a mild direction (the west), average temperatures during this stormy period were about 2 degC above average in Reading.

Rainfall

But being an inland site, winds in Reading are necessarily less than those reported in coastal locations – even along the English Channel just to the south during this period. Rainfall was the main feature of the weather in Reading; persistent falls over a short period were to have a major impact on travel in and around Berkshire (and, indeed, in many other parts of the British Isles) from about 23rd December onwards.

The fall of 29 mm on the 23rd was the greatest 24-hour fall in December since 19th December 1995 and only 1989 (35.8 mm),  1954 (43.2 mm) and 1919 (33.5 mm) have had a wetter December day at the University since 1908.

Around Reading, due to the dry conditions earlier in the month, the main disruption did not occur until after Christmas as the ground was still, relatively, dry. However, just a short distance to the south and south-east falls of 40-75 mm fell in parts of Hampshire, Surrey and Kent on the 23rd-24th, resulting in rivers bursting their banks and consequently a power loss at Gatwick Airport (for example).

Reading daily rainfall totals, December 2013-January 2014

In the 22-day period 15th December to 5th January a total of 152.3 mm of rain fell at the University. Climatological averages of rainfall in Reading during 1981-2010 are 63.0 mm in December and 60.4 mm in January, with the annually-average total amounting to 634.6 mm.

While the total rainfall in December 2013 (106.0 mm) was less than in December 2012, the 22-day total was the greatest such fall since October-November 2000 when 157 mm fell in a 22-day period. Indeed, by the 7th January the fall during the (26-day) wet spell had reached the equivalent of a quarter of the annual average rainfall total in Reading.

Looking back over the records, we find there have been seven such wet periods:

  1. November-December 1929 when 178 mm was the greatest 22-day rainfall total. This spell included the second wettest November on record. By the end of the November the Thames was approaching a state of flood and it duly burst through its banks in several places. 164 mm fell in an 18-day period and not until mid-December did it turn dry in Reading for a few days. Seven days recorded in excess of 10 mm of rain in Reading.
  2. October-November 1940 (171 mm in 22 days). November 1940 was another wet November although the final week was a dry one. A fall of 30.2 mm on 13th November was one of the wettest November days on record at the University.
  3. October 1949 (154 mm in 22 days). There were many dry days in this spell –three-quarters of the rain fell on five separate days allowing for some drying out in between events.
  4. October-November 1951 (162 mm in 22 days). A four-week wet spell from the final four days of October onwards led to the wettest November on record in Reading with by far the wettest November day on record – a fall of 38.4 mm.
  5. June 1971 (155 mm in 22 days). This was an unusual summer wet spell. June 1971 was a cool and wet month – the only June on record to surpass a total of 150 mm. However, no measurable rain fell during the first week, nor between the 20th and 24th; 10 mm or more fell on four days with 54.7 mm falling on the 10th during a spell of almost 21 hours of rain that day. Some flooding occurred around Reading as a result of this wet spell.
  6. October-November 2000 (157 mm in 22 days). Much of this fell in a 10-day spell beginning on 28th October. Flooding along the Thames was relatively minor compared to some past events – but further heavy rain on several days in December caused more flooding by mid-December.
  7. December 2013-January 2014 (152 mm in 22 days). This led to river flooding in places around Reading into early January in particular.

Note that the statistics presented in these seven notes above all refer to the University of Reading weather station.

Flooding

Flood warnings in the Reading area, 1621 UTC 7th January 2014

The figure above shows the extent of flood warnings in the Reading area, as of 1621 UTC on 7th January 2014. The image is from the website of the Environment Agency, http://www.environment-agency.gov.uk/homeandleisure/floods/142151.aspx. Flood alerts are shown in orange and flood warnings in red.

Although there were small pockets of flooding to the south of Reading as a result of the rainfall on the 23rd December, due to the dry spell of late November-early December for many in the Reading area flooding did not occur until the ground became progressively saturated after Christmas and into 2014. This led to large amounts of runoff into the rivers passing through the town, causing some travel disruption on the roads and railways – in particular once the Sonning Bridge was closed, restricting traffic flow across the River Thames.

Written: 7th January 2014


The summer of 2013 in Reading

The first five months of the year in Reading had been cool – with an average temperature 1.8 degC below normal. After the coldest May in Reading for 17 years, June continued in the same vein – being 0.9 degC colder than average and the coldest June (along with June 2012) since 1991. However, in contrast to June 2012, June 2013 was dry with just 21 mm of rainfall making it the driest June since 2010. The lack of rain was not associated with a lower-than-normal cloud cover and June was slightly duller than average – and duller than in both April and May this year.

But things were to change in July.

Fig 1. The University of Reading's Campbell-Stokes sunshine recorder

The weather station at the University of Reading recorded its third sunniest month in July since sunshine records began in 1956. A total of 297.6 hours of bright sunshine was measured with the station’s Campbell-Stokes sunshine recorder – only a few hours less than were recorded in July 1959 (298.2 hours) and June 1975 (305.6 hours). With little rainfall on the 31st it had seemed at one stage as if the ‘magical’ total of 300 hours might have been exceeded – but it was not to be, as the sky remained rather cloudy throughout the last day of the month and only 24 minutes of sunshine occurred. Cloudy skies on the 2nd and 3rd also helped to keep the total lower than it might have been.

During the peak of the sunny conditions there were 15 consecutive days (from the 5th to the 19th) with over 9 hours of sunshine – the longest such spell in July on record. In addition, from the 7th to the 13th every day had over 12 hours of sunshine – this has only been bettered (by longer such spells) in any summer month on three occasions since 1956.

Fig 2. Daily maximum and minimum temperatures, summer 2013; solid lines show the corresponding climatological values

With an average temperature of 19.5 °C it turned out to be the warmest July since July 2006 (which was the hottest July of the past 106 years at the University). Temperatures reached 30 °C on three days, peaking at 31.5 °C on the 22nd – the highest temperature in Reading since July 2006. There were also some warm and humid nights during the month – overnight 22nd/23rd the temperature did not fall below 17.6 °C in Reading. Daytime maximum temperatures were conspicuously above average from the 4th to the 27th – see Fig.2 which shows the daily maximum and minimum temperatures recorded at Reading – and also the climatological values (solid lines).

July’s summery weather was the result of a persistent area of high pressure which remained over us for almost three weeks. After 1.8 mm of rain on the 2nd, the raingauge at the University of Reading remained dry (apart from some light spots of rain during thundery weather on the 23rd) until the 24th – this was the longest period of summer drought at the weather station since August 1995.

After the high pressure moved away from southern England we had five days with thunderstorms in and around Reading. However, rainfall amounts were mostly slight apart from a fall of 15 mm on the 29th; the total fall for the month of 34.9 mm was less than average and made it the driest July since 2006.

Conditions in August continued the dry theme – just 20.8 mm of rain fell in Reading, the driest month since March 2012 and the driest August since 2003 (when 14.2 mm fell). There were no thunderstorms in August and the wettest day (the 5th) brought just 5.9 mm of rain as the town missed the heavy falls that affected places to the east.

The mean August temperature was 0.6 degC above average although.,apart from on the 1st (when 32.0 °C was recorded – the highest temperature of the summer in Reading), the temperatures during August were never that far away from the average. August was the warmest since 2004 – but this was more a reflection of the lack of hot Augusts in recent years than in any prolonged heatwave this month. 224 hours of bright sunshine in August made it the sunniest August since 2005.

So how does this summer compare to others?

Overall it was the warmest, sunniest and driest summer (June-August) since 2006 (which in turn was 1.3 degC warmer). Air pressure was on the high side overall (the anomaly was +2.6 mb compared to the 1981-2010 average), consistent with the reduced rainfall total given the lack of heavy thundery downpours – and was also associated with an increased tendency towards a flow from a NE’ly direction at times.

Fig. 3 Hodograph of the 0900 GMT wind direction in summer 2013 (values are percentages)

Poulter (1962) [Poulter R.M. ( 1962), The next few summers in London, Weather, 17, 253-257] devised a (possibly controversial) summer index as a way of combining the elements of average temperature, rainfall and sunshine into a single value to denote the ‘goodness’ of the summer. Essentially the range of these three elements are scaled to value of about 70, and then the elements are combined such that sunshine and temperature make a positive contribution, and rainfall a negative one, to the index.

Using summer totals of sunshine and rainfall and mean summer air temperatures for the period 1968-2012 (during which the ranges from best to worst summer were 221.7 mm, 396.6 hours and 3.85 degC respectively), we can derive the Poulter Index (P) for Reading as follows:

P = 18.18T + S/5.67 –R/3.65

where T is the average temperature (°C), R is the total rainfall (mm) and S is the total sunshine (hours). Allowing for an average summer temperature difference of 0.6 degC between Whiteknights and London Road we can calculate the Poulter Index as shown in the figure for the period 1959-2013:

Fig 4. The Poulter summer index for Reading

This confirms that the summer of 2013 in Reading might be considered as the best since 2006 – and similar in some ways to the summers of 2003, 1989, 1983, 1975 and 1959. It was certainly an improvement on those of 2007-2012.

Acknowledgements

Thanks to Mike Stroud and John Lally in particular for making the observations every day.

2012-2013 – a long winter and a cold Easter

The winter of 2012-13 definitely extended into late March (and, as I write this, into early April). Figure 1 (courtesy of Mike Stroud, our meteorological observer) sums up why it failed to feel like spring towards the end of the month.

Figure 1. March 2013 temperatures in Reading

Shown in red are the daily values of the maximum and minimum air temperatures for each day, while the green points show the corresponding grass minimum temperatures. The latter are the readings used to define the occurrence of  ‘ground frost’. Comparing the air temperatures with the climatological averages (the blue lines) it can be seen that just two days were warmer than average by day (in fact, the 5th might be described as being very warm for early March).

However, just six days after this very warm day (on the 11th) the maximum temperature in Reading rose to only 0.9C – the lowest such value in March since 1986 (when it remained below 0C all day on the 1st). [11th March 2013 was also the first of two days when the Channel Islands had one of their worst snowstorms on record, giving the greatest March snow depth at Guernsey Airport in their entire record (which goes back over 50 years).]

But things didn’t really get any better. Maximum temperatures in Reading on the 23rd (1.0C), 24th (1.0C) and 25th (1.1C) have never been this low in the 108-year station record in the final 11 days of March – and to have three consecutive days with such low values at this time of year is truly remarkable. Fortunately Reading missed out on the severe snowstorms that affected areas of N England and Scotland around this time – but we still had 11 days with snow or sleet falling during the month, a March total equalled (since 1908) only in March 1937.

With March (and Easter at the month’s end) being a time when gardeners like myself start trying to prepare their vegetable beds for spring sowing, the frequency (and severity) of ground frosts during the month was of no help at all. Four nights dipped below -10C at grass tip level (two of those being in the final three days of the month) – not since 1985 have there been four ground frosts this sharp in March. These cold nights along with a wet soil – March was another wet month in the 12-month spell of weather that began in April 2012 – and a lack of warmth by day led to cold soil conditions. At 0900 GMT on 31st March 2013 the soil temperatures (and their anomalies with respect to the 1981-2010 average value for that date) were as follows:

  • 5 cm depth: 0.0C (anomaly -7.7 degC)
  • 10 cm depth: 0.7C (-6.5 degC)
  • 20 cm depth: 2.1C (-5.5 degC)
  • 30 cm depth: 4.2C (-3.9 degC)
  • 50 cm depth: 4.6C (-3.0 degC)
  • 100 cm depth: 5.5C (-2.4 degC)

Such temperatures are ones that are more typical of a cold spell in mid-winter!

Even allowing for the fact that Easter was quite early this year, air temperatures were remarkably low over the four-day period (Friday-Monday) as shown in figures 2 and 3. In these figures the black triangle denotes the value for 2013 with data plotted against the day number of the year representing Easter Sunday.

Figure 4. Easter holiday mean temperature in Reading 1908-2013

Figure 2. Easter holiday mean temperature in Reading 1908-2013

Figure 4. Easter holiday minimum temperature in Reading 1908-2013

Figure 3. Easter holiday minimum temperature in Reading 1908-2013

Overall during March the average daily maximum temperature in the air was 4.5 degC below average while the night minimum temperature was 3.1 degC below average. The month as a whole was the coldest March since 1962 – and since 1908 only March 1917 has also been colder (once allowance is made for the fact that readings before 1968 were made at the London Road campus – a slightly warmer site).

More remarkable is the fact that the average maximum temperature for March 2013 was the lowest for March since Reading’s observations began in 1908. Local records from nearby sites (S Burt, pers. comm.) suggest that we need to go back to before 1871 to find a lower March mean maximum temperature.

Why was it so cold – figure 4 suggests a rather unusual flow from an easterly direction during the month…

Figure 2. March 2013 MSL pressure at 0900 GMT

Figure 4. March 2013 MSL pressure at 0900 GMT

…and this flow was persistent as shown by the winds observed by the Reading automatic weather station (observations every 5 minutes) in figure 5.

Figure 3. Wind hodograph for reading in March 2013

Figure 5. Wind hodograph for reading in March 2013

With such a persistent flow from the east the cold can be understood – especially as Scandinavia and N Europe were at times 8 degC colder than average during March. The fact that March was a dull month – the dullest since 2001 – is also understandable. But it was also a wet one – fronts often tended to bring precipitation from a southerly direction while the winds blew from the east around low pressure to the south. This pressure distribution was put down to the jet stream taking a more southerly track through Europe during the month.

The end of March marked the 12th month since the dramatic change to wet weather in April 2012. These 12 months have given Reading 913.9 mm of precipitation – or 44 per cent more precipitation than average.